When signals are transmitted wirelessly through a nonlinear channel, they will mix with each other, self-mix and or mix with noise components in the channel creating unwanted signal components that create interference. Active intermodulation products are produced in the active components, such as, for example, amplifiers and mixers, during processing of wireless signal. Passive intermodulation products are generated after the amplifier by interaction with external materials such rusty components and external reflective surfaces.
FIG. 1 shows a typical example of an amplifier transfer curve. This type of transfer function is also generally representative of any non-linear channel. The 1:1 curve is known as the linear region of the amplifier and when the amplifier is driven beyond this range, the deviation from linear is experienced. The curved line is the compression curve of the amplifier. This can be modelled as a power series expansion such as aX+bX2+cX3+dX4 . . . and power series expansions like this predict the higher order intermodulation products. All signals convolved with this transfer function are effectively multiplied together and this creates Intermodulation Products. Intermodulation Products (IMP) are of the order 2nd, 3rd, 4th etc. In most cases, the even order IMPs will not fall inband, but if they do, they can be handled as described herein for odd order IMPs.
Interference Environment
When high power out of band signals (source signals) are incident on the RF front end of a wideband receiver such as a cell phone, or cell phone base station, intermodulation products (IMPs) can be generated in the analog components of the receiver. These intermodulation products (IMPs) can have significant power. This is the case for cellular handsets which must receive the entire telephone band because the receiver does not have a prior knowledge of the channel to which it will be assigned. The channel selection is done after frequency down conversion because adjustable channel selection at RF is not practical and the insertion losses would be unacceptable.
Passive IPMs are created in the microwave components and structures after the transmitter filter and are thus not removed by the system filters and can land inband of the receive signals.
In cellular base stations, selective filters provide for rejection of adjacent channels that are not of interest, but passive IMPs can be generated by signals from the individual service provider and or other service providers because the source signals are out of band of the passband of interest, but the passive IMPs may not be and when they fall inband of the signal of interest, significant interference can be realized. The wideband LTE, Gen 4 and Gen5 systems will make the passive IMP problem even more severe.